Liquid crystals are used in a wide variety of devices, including visual display devices. The property of liquid crystals that enable them to be used, for example, in visual displays, is the ability of liquid crystals to transmit light on the one hand and to scatter light and/or absorb it (especially when combined with an appropriate dye) on the other, depending on whether the liquid crystals are in a relatively free, that is de-energized or field-off state, or in a relatively aligned, that is energized or field-on state. An electric field selectively applied across the liquid crystals may be used to switch between the field-off and field-on states.
There are three categories of liquid crystal material, namely cholesteric, nematic and smectic. The present invention relates in the preferred embodiment described hereinafter to the use of liquid crystal material which is operationally nematic. By "operationally nematic" is meant that, in the absence of external fields, structural distortion of the liquid crystal is dominated by the orientation of the liquid crystal at its boundaries rather than by bulk effects, such as very strong twists (as in cholesteric material) or layering (as in smectic material). Thus, for example, a liquid crystal material including chiral ingredients which induce a tendency to twist but which cannot overcome the effects of the boundary alignment of the liquid crystal material would be considered to be operationally nematic.
A more detailed explanation of operationally nematic liquid crystal material is provided in U.S. Pat. No. 4,616,903, issued Oct. 14, 1986, entitled ENCAPSULATED LIQUID CRYSTAL AND METHOD, assigned to Manchester R&D Partnership, the disclosure of which is hereby incorporated by reference. Reference may also be made to U.S. Pat. No. 4,435,047, issued Mar. 6, 1984, entitled ENCAPSULATED LIQUID CRYSTAL AND METHOD, assigned to Manchester R&D Partnership, and which disclosure is also hereby incorporated by reference.
NCAP liquid crystal and devices using NCAP liquid crystal are also described in the above-identified U.S. Pat. No. 4,435,047. A functional NCAP liquid crystal device may consist of NCAP liquid crystal sandwiched between two electrode-coated substrates. The substrates may be polyester (PET) coated with indium tin oxide to form electrodes. The encapsulated NCAP or film may comprise a containment medium containing plural volumes of operationally nematic liquid crystal. The plural volumes may be discrete or interconnected cavities or capsules. The interconnecting channels or passageways may also contain liquid crystal material. This structure is described in more detail in U.S. Pat. No. 4,707,080, issued Nov. 17, 1987, entitled ENCAPSULATED LIQUID CRYSTAL MATERIAL, APPARATUS AND METHOD, assigned to Manchester R&D Partnership, the disclosure of which is hereby incorporated by reference.
A voltage source may be connected between the electrodes to selectively apply an electric field across the liquid crystal material. As is known, the liquid crystal material will scatter and/or absorb light in the field-off state and transmit light in the field-on state. Thus, the liquid crystal material or film will be clear in the field-on state and cloudy or hazy (in the absence of a pleochroic dye) in the field-off state.
The NCAP film may be used in the construction of windows and the like. Such apparatus are described in U.S. Pat. No. 4,749,261, issued June 7, 1988, entitled SHATTER-PROOF LIQUID CRYSTAL PANEL WITH INFRARED FILTERING PROPERTIES, assigned to Taliq Corporation. A window may be fabricated by laminating the electrode-coated substrate that supports the NCAP film to a window surface, for example glass or a sheet plastic, by means of an optically-transparent adhesive or interlayer. One of the more commonly used glass interlayers is polyvinylbutyral (PVB). Others are ethylenevinyl acetate (EVA) and polyurethane.
PVB and EVA are thermoplastic film adhesives as opposed to liquids. As such, they offer convenient handling and processing. These interlayer materials also can provide safety glazing and impact resistance properties. Other advantages include: clearness, low haze, environmental stability, and ultra-violet light absorption.
The lamination of a NCAP film substrate to a glass or plastic surface may, however, adversely effect the electro-optical performance of the film. That is, portions of the film may no longer scatter light as effectively in the field-off state. Thus, those portions would transmit light more specularly and be clearer in the field-off state. This is thought to be caused by mechanical stresses applied to the NCAP film during the lamination process. Such mechanical stresses are believed to cause a change in the shape and/or structure of the cavities, capsules or interconnecting passageways. Additionally, such stresses may cause a flow of liquid crystal, for example to the film's surface, through the passageways or pores in the containment medium. This phenomenon, called "stress clearing", degrades the film's electro-optical performance.
Accordingly, an object of the present invention is to provide an encapsulated liquid crystal material that is more resistant to mechanical stress.
A more specific object of the present invention is to provide a NCAP film incorporating a polymer additive that forms a wall within volumes of the containment medium containing liquid crystal material.